PCT/EP2009/058041 discloses separate production of crosslinking agent masterbatches with the aim of introducing these continuously into the parent mixtures produced batchwise. Although this process has the advantage that it can produce crosslinkable rubber mixtures in a manner which is more practical and which provides better performance, the quality of dispersion of the crosslinking agent masterbatch in the rubber mixture cannot be demonstrated—or if it can be demonstrated this is possible only offline.